1. Field of the Invention
The present invention pertains to a method for improving the quality and a fidelity of seismic images created by the process of "Seismic migration." More particularly, the present invention deals an automatic way to calculate a "model based migration aperture," which determines the range of the seismic data that is most relevant for imaging a selected area in depth.
2. Description of the Prior Art
Seismic data is gathered by generating seismic pulses into the earth and detecting the reflected, refracted, or diffracted waves with seismic receivers. A two-dimensional seismic profile (or spread) consists of a single source (shot) and a series of receivers located uniformly along a line. Such a configuration of source and receivers is called common-shot ("CS") profile. Typical distance between receivers is in the order of several tens meters, where for about hundred receivers, the spread length may extend to several kilometers. Continuous profiling results from moving a seismic profile by constant steps, usually equal to the receivers interval, along a seismic line. Thus, an individual seismic line may extend for distances of several tens kilometers.
A CS seismic record is a set of traces, each representing a receiver, for a given shot. The recorded amplitudes represent the acoustic pressure (at sea) or the particle velocity (on land) as a function of time. Seismic traces typically extend from five (5) seconds ("sec") to ten (10) sec in time since hydro carbon reservoirs rarely occur below seven (7) kilometers. ("km") in geologic basins, where velocity of the propagating waves ranges from 1.5 km/sec to seven (7) km/sec. The detected signals contain frequencies from a few hertz to a few hundred hertz, and are sampled at one, two, four or eight millisecond ("msec") intervals.
In the seismic reflection methods, the data is commonly rearranged from CS gathers to common-midpoint ("CMP") records. A CMP record consists of traces corresponding to source-receivers pairs located symmetrically about a CMP. Thus a CMP represents a point where the locations of the shot and the receivers coincide. It is also called the zero offset point. The number of shot-receiver pairs used in a CMP record determines the subsurface coverage (fold).
The subsurface of the earth is assumed to consist of a sequence of geological layers separated by curved interfaces. For seismic imaging, the most significant material parameter is the velocity of the propagating waves. When a seismic wave travels down through the earth and hits an interface, separating two different material regions, a portion of the wave is reflected back to the earth surface and detected. The recorded pulses within the seismic traces yield an image of the subsurface indicating major geological interfaces (reflections). This image is applicable only as long as the interfaces are continuous and horizontally flat and when no lateral velocity variations within the layers exist. However, the subsurface of the earth might be complex, especially in areas of hydrocarbon traps, where the interfaces are discontinuous curved surfaces broken by faults. In such areas the reflections are misplaced and the seismic image is distorted by diffractions. For this reason, a process of migration is routinely performed.
Migration is a process which maps seismic pulses which are recorded in the time domain into a depth domain through a wave equation and a suitable velocity field. Specifically, seismic migration is an inversion operation involving rearrangement of seismic information elements so that reflections and diffractions are plotted at their true locations. Originally, migration was performed by hand on interpreted seismic data. In current practice, computer operations are performed typically on uninterpreted data using some form of, or approximation of, the wave equation by way of one of several solutions: solution in the space time domain by a finite-difference method; solution in the integral form (Kirchoff migration); solution in the frequency domain; or solution by a combination of the previously mentioned domains. Examples of migrating techniques are shown in U.S. Pat. Nos. 4,464,737; 4,479,205; 4,745,585; 5,198,797 and U.S. Statutory Invention Registration No. H482.
More specifically, seismic data migration is the inverse of seismic wave propagation. Under Huygen's principle, as a seismic wave propagates through a subsurface, each point it passes becomes a secondary point source. These secondary point sources radiate the energy in all directions. The arrival times of the energy which returns to geophones at the surface from such a secondary point source forms a "hyperbolic event" often referred to as a curve of maximum convexity or a diffraction curve.
A stacked seismic section approximates the recording of all of the hyperbolic events from all of the points the wave passed through for some finite period of time. The reflections seen on a stacked section are simply the areas where the hyperbolic events reinforce one another. A stacked seismic section is always out of focus and is often highly distorted. The focus deteriorates with depth because the curves of maximum convexity become more flattened. The distortion is most evident on the dipping data, which appear down dip from their proper spatial locations. Migrating seismic data is an attempt to reverse the focus distortion process of the propagating seismic waves recorded. Seismic migration focuses all of the data and relocates the dipping data to their proper spatial locations.
In principle, all seismic migration algorithms "collect and combine" the data along the curve of maximum convexity for every point within the subsurface and output the data at its apex. The term "collects and combines" in this context covers everything from simple stacking to frequency/phase filtering with complex weighing, etc. Collection and combination may occur in one of several different domains.
The aperture of a migration algorithm is the limit to which the algorithm reaches out to collect the data along the curves of maximum convexity. Almost all Kirchoff or summation migration algorithms require the aperture to be time variant. Some allow the aperture to be space variant. Many allow the aperture to be asymmetrical and therefore require the specification of the aperture for both the left and right hand sides of the operator. Some algorithms allow and/or require that the aperture be frequency dependent.
In Kirchoff integral migration algorithms the maximum apparent dip and the maximum operator length usually determine the aperture. The maximum apparent dip parameter restricts the aperture by having the algorithm collect data along the curves of the maximum convexity curve up to the point where the slope reaches the maximum apparent dip. Since the curvature of the maximum convexity curves decrease with depth, their slopes may never reach the maximum apparent dip in the deeper portion of the section. The maximum operator length acts as a fail safe provision which limits the absolute length of the operator regardless of slope. The maximum apparent dip and maximum operator length are typically selected by visual inspection and, thus, have heretofore relied totally on user judgement.
The migration aperture is determined by the size (usually specified as a length) of the migration operator. The terms "migration aperture" and "migration operator size" often are used interchangeably. Migration aperture, however, evokes the analogs between the processes of seismic migration, achieving accurate spatial location and optical focusing to thereby achieve adequate fidelity of the migrated results. In this context, reasonable fidelity means that the migrated data retain their relative amplitudes, phase, and frequency contents. Thus, a seismic migration process "migrates" dipping data to their proper spatial locations and simultaneously focuses the dipping and non-dipping data. An adequate migration aperture is large enough to "migrate" dipping reflections to their proper spatial locations and to focus them with reasonable fidelity.
In generals as the migration aperture is increased, the ability of the algorithm to "migrate" dipping data to their proper spatial locations with reasonable fidelity is improved. However, since seismic data sets are spatially discrete (i.e, each trace is recorded separately), the aperture of migration algorithms must be restricted to avoid the degrading effects of spatial aliasing. This is a fundamental tradeoff in all migration processes. An aperture must be large enough to properly migrate the dipping data and yet not so large as to introduce artifacts from spatial aliasing.
The requirements to migrate seismic data properly, regardless of the technique used, include an appropriate velocity model, a good migration algorithm, and an adequate migration aperture. Although there are many different migration algorithms, the basic principles involved in the specification of an adequate migration aperture are the same. The specification of an adequate migration aperture can be separated into two parts that insure the proper spatial location and fidelity of the migrated results, respectively. Therefore, as previously stated, an aperture must be specified that is large enough to properly migrate the dipping data and yet not so large as to introduce artifacts from spatial aliasing.
Even when an exact velocity model and the best migration algorithm are used, an inadequate migration aperture will cause the resulting migrated seismic section to be seriously flawed. An inadequate migration aperture can reduce the relative amplitude of dipping data, distort the frequency and/or phase content of the reflections, or, in extreme cases, fail to position dipping data in their proper spatial locations. Furthermore, these serious flaws often go undetected.
Typically, once the aperture is determined for a given section or volume, the aperture remains constant throughout the entire migration process. Since the characteristics of the subsurface data tend to vary drastically throughout a data section or volume, an aperture selected for one part of the data section may not be optimum for another part. Therefore, typically an aperture is selected to be large enough to be appropriate for all data in the data set, requiring an enormous amount of processing time to complete the migration of all the data in the data set.
It is, therefore, a feature of the present invention to automatically calculate a migration aperture size from a seismic data set to be migrated, rather than rely on user judgement to determine the aperture size.
It is another feature of the present invention to provide a method of migrating seismic data using a different migration aperture for every part of seismic data or a seismic data volume.
It is yet another feature of the present invention to provide a method for optimizing seismic data migration by limiting the area to be migrated in accordance with a displacement section of the data.
It is still another feature of the present invention to provide a means to verify the accuracy of a velocity section of an area of interest.